1. Field of the Invention
The present invention relates to electrical-test equipment used for tracing conductors and identifying electrical circuit elements.
2. Discussion of Related Art
Electrical work often requires identifying elements of a circuit or tracing a circuit behind a wall or other obstruction. For example, an electrician may wish to determine which circuit breaker is attached to a particular wall outlet so that repairs may be made. By identifying the proper circuit, the electrician can de-energize just a single circuit before performing the repairs. Typically, the electrician prefers not to shut down equipment attached to other circuits. Alternatively, the electrician may wish to trace a hidden wire along a wall to locate a convenient place to add another outlet.
Some devices for locating and identifying electrical circuits use a transmitter and a receiver. A transmitter 10 induces a current signal on the circuit in question. A receiver 100 senses the induced signal.
FIG. 1 illustrates how a transmitter and receiver are used with a power distribution system. A transmitter 10 is physically connected to a circuit 11 in question. The transmitter 10 may be connected to the circuit 11 by, for example, plugging the device into an outlet (as shown) or using jumper wires. The transmitter 10 induces an electrical current signal in the circuit 11 in question. The circuit 11 radiates electromagnetic radiation 20 along its path. The receiver 100 receives the electromagnetic radiation 20 emanating from the circuit 11 in question.
The receiver 100 may be used to identify a circuit breaker fuse 13 connected to circuit 11 or may be used to trace hidden wires of circuit 11. To identify a circuit breaker, the electrician scans the receiver 100 over a circuit breaker panel 12 containing multiple circuit breaker fuses 13 and 14. Circuit breaker fuse 13 is directly connected to circuit 11 while circuit breaker fuses 14 are connected to other circuits 15. As the receiver 100 passes over circuit breaker fuse 13, the receiver 100 alerts the electrician. To trace a hidden wire, the electrician passes the receiver 100 over the area suspected of concealing the circuit 11. The receiver 100 provides the electrician with a signal strength indication of received electromagnetic radiation 20.
Some devices for identifying and tracing electrical systems use low-frequency, short duration signals. They use the line frequency of 50 Hz or 60 Hz. The transmitter sends a short duration pulse that lasts for approximately 10 microseconds. Due to the nature of the transmitted pulse, the frequency spectrum is very wide and an associated receiver is required to sense a wide-bandwidth radiated signal. For examples of low-frequency, short duration pulse transmitters and wide-bandwidth receivers, see U.S. Pat. Nos. 4,556,839, 4,906,938, 5,497,094, and 5,969,516, herein incorporated by reference.
Other devices for identifying and tracing electrical systems modulate a signal on a high-frequency carrier. Their carrier frequencies range from approximately 3950 Hz to approximately 200 kHz. A high-frequency carrier has the advantage that the transmitter signal easily couples to the receiver. For examples of transmitters and receivers sending and sensing carrier signals modulated on a high-frequency, see U.S. Pat. Nos. 4,491,785, 4,642,556, 4,801,868, 5,418,447, 5,422,564, and 6,163,144, herein incorporated by reference.
Known devices either: (1) use a manual calibration system that requires the electrician to adjust the sensitivity of the receiver; or (2) require the electrician to remember the strongest signal sensed as a scan is performed.
As described below, these known devices give false-positive indications for several reasons. For example, (1) the signal from the transmitter couples to adjacent circuits; (2) a load on another circuit masquerades as the transmitted signal; and (3) the electrician fails to properly calibrate the device.
A receiver 100 can give a false-positive indication when a signal 20 from a transmitter 10 couples to adjacent or neighboring circuits 15. Electromagnetic radiation 20 radiates from the target circuit 11 carrying the transmitted signal to neighboring circuits 15 thus inducing current on the neighboring circuits. Coupling from the target circuit 11 to neighboring circuits draws energy away from the target circuit 11. The magnitude of the signal coupled to a neighboring circuit 15 relates to the transmitted signal's carrier frequency. The higher the carrier frequency, the more easily the signal couples to other circuits. A change to the carrier frequency causes a proportional change to the magnitude of the coupled signal. The non-target neighboring circuits 15 re-radiate the coupled modulated signal and thus may lead to false-positive indications.
A receiver 100 can give a false-positive indication when a load on another circuit 15 masquerades as the transmitted signal. Loads on other circuits 15 might generate noise that may be miss-interpreted as a signal from the transmitter 10. For example, power modulating devices, such as switching power supplies, light dimmers, and motor controllers, generate noise that a receiver 100 might erroneously identify as a signal from the transmitter 10. Some power modulating devices reference the power line voltage and frequency when generating power. Consequently, these devices may create extraneous current noise at multiples or harmonics of the power line frequency. A receiver 100 might not be immune to this current noise from active loads and may erroneously determine that this noise is a signal sent by the transmitter 10.
To address the noise immunity problem described above, some transmitters use a modulation scheme that the receiver automatically recognizes. Some devices modulate a low-frequency signal on a high-frequency carrier. These devices rely on the electrician to perceive the difference between a transmitter's signal and noise generated by loads. These devices offer visual and audio indicators that pulse at the low-frequency signal rate. See, for example, U.S. Pat. Nos. 4,642,556, 5,418,447, 5,422,564 and 6,163,144, herein incorporated by reference. High-frequency carriers used by these systems more often exhibit detectable intercircuit coupling. By the selection of a high-frequency carrier, these systems inherently fail to address the problem of a transmitted signal on a target circuit 11 coupling to adjacent circuits 15.
A receiver can also give a false-positive indication when the electrician fails to properly calibrate the device. To calibrate some devices, the electrician manually adjusts the gain of the received signal using a sensitivity adjustment. By reducing the sensitivity, fewer signals are detected. The electrician take readings from each of the candidate elements while continually adjusting the calibration control until only one indication is obtained. Similar devices take a different approach that includes a thermometer-type visual display and variable-volume audio indicator. The electrician is instructed to remember the largest signal observed and to take this signal as the identified target circuit. See, for example, U.S. Pat. No. 6,163,144, herein incorporated by reference. These system rely on human experience and skill to properly detect circuits.
Thus, there is a desire and need for a device and method capable of tracing conductors and identifying electrical circuit elements with a reduced false-positive error rate.